The present invention relates to a garnet film for magnetic bubble memory devices, and more particularly to a magnetic bubble element garnet film ideally suited for supporting small magnetic bubbles with a diameter of 0.4 micron or less.
It is known that, to reduce the bubble diameter in garnet material used in magnetic bubble memory devices, the saturation induction 4.pi.Ms need only be increased. To reduce the bubble diameter down to 0.4 micron or less,4.pi.Ms must be set at no less than about 1900G. However, the majority of the saturation magnetization (Ms) of iron garnet arises through the difference between the magnetization of ferric ions occupying tetrahedral sites (Fe.sup.3+ ; three ions in the composition formula) and the magnetization of ferric ions arranged in the opposite direction occupying octahedral sites (Fe.sup.3+ ; two ions in the composition formula). Magnetic garnet films with the desired 4.pi.Ms have hitherto been obtained through the substitution for tetrahedral iron ions (Fe.sup.3+) of ions such as gallium (Ga.sup.3+), aluminum (Al.sup.3+), silicon (Si.sup.4+), or germanium (Ge.sup.4+) ions, which are strongly selective for tetrahedral sites.
It is therefore possible to reduce the bubble diameter by reducing the amount of ions such as Ga.sup.3+, Al.sup.3+, Si.sup.4+, and Ge.sup.4+ that replace the tetrahedral ferric ions (Fe.sup.3+), and thereby increasing the 4.pi.Ms. For example, in (SmLu).sub.3 Fe.sub.5-Y Ga.sub.Y O.sub.12 type garnet, a magnetic bubble garnet film with a 4.pi.Ms of approximately 1800G is formed by reducing Ga amount y to zero (AIP Conference Proceedings, No. 29, 105-107, 1975). However, because the amount of tetrahedral ferric ion substitution in this garnet system cannot be made any smaller than this, it is impossible to make the 4.pi.Ms any larger.
Another possible method of increasing 4.pi.Ms is to substitute octahedral ferric ions with non-magnetic ions such as scandium (Sc.sup.3+) that are strongly selective for octahedral sites. It is known, for example, that by increasing the Sc content Y from 0 to 0.7 in Y.sub.3 Fe.sub.5-Y Sc.sub.Y O.sub.12, the 4.pi.Ms at absolute zero point increases about 50% (J. Appl. Phys. 37, 1408-1415, 1966). It is thought that substituting the octahedral ferric ions with non-magnetic ions will suffice for increasing the 4.pi.Ms value above the 1800G obtained in (SmLu).sub.3 Fe.sub.5 O.sub.12 above. However, as shown in FIG. 1, actual measurements at room temperature (25.degree. C.) of the 4.pi.Ms of garnet film in which the octahedral ferric ions have been substituted with Sc.sup.3+ show that 4.pi.Ms hardly increases even when the Sc content y is increased. As shown in FIG. 2, this is because the Curie temperature (T.sub.c) declines as the amount y of Sc increases; as a consequence of this, it is believed that the magnetic interaction between the ferric ions weakens, which is why the 4.pi.Ms fails to increase.